The present invention relates generally to data communication systems and more particularly relates to a receiver for use in a code shift keying based direct sequence spread spectrum communication system.
The use of spread spectrum communications techniques to improve the reliability and security of communications is well known and is becoming more and more common. Spread spectrum communications transmits data utilizing a spectrum bandwidth that is much greater than the bandwidth of the data to be transmitted. This provides for a more reliable communication in the presence of high narrowband noise, spectral distortion and pulse noise, in addition to other advantages. Spread spectrum communication systems typically utilize correlation techniques to identify an incoming received signal.
Spread spectrum communications systems are commonly used in military environments to overcome high energy narrowband enemy jamming. In commercial or home environments it may be used to achieve reliable communication on noise media such as the AC powerline. In particular, certain home electrical appliances and devices can potentially be very disruptive of communications signals placed onto the powerline. For example, electronic dimming devices can place large amounts of noise onto the powerline since these devices typically employ triacs or silicon controlled rectifiers (SCRs) to control the AC waveform in implementing the dimming function.
A communication medium such as the AC powerline may be corrupted by fast fading, unpredictable amplitude and phase distortion and additive noise. In addition, communication channels may be subjected to unpredictable time varying jamming and narrowband interference. In order to transmit digital data over such channels it is preferable to use as wide a bandwidth as possible for transmission of the data. This can be achieved using spread spectrum techniques.
One common type of spread spectrum communications, called direct sequence spread spectrum, is generated by first modulating the digital data and then multiplying the result with a signal having a particularly desirable spectral properties, such as a PN sequence. The PN sequence is a periodic sequence of bits having a particular period. Each bit in the sequence is termed a chip. The sequence has the property of having very low autocorrelation for delays larger than one chip. In some systems, the PN sequence is replaced by a chirp waveform. Several techniques are available for the transmitter to modulate the data signal, including biphase shift keying (BPSK) and continuous phase modulated (CPM) techniques. Minimum shift keying (MSK) is a known variation of CPM.
The spread spectrum receiver is required to perform synchronization that is commonly implemented using an acquisition method in combination with a tracking loop or other tracking mechanism. In a noisy unpredictable environment such as the AC powerline, the tracking loop typically fails frequently causing loss of information. Communication systems to overcome these problems are large, complex and expensive. In addition, these systems typically succeed at transmitting only one or more bits per symbol.
Receiving and demodulating signals that have been subject to PN modulation requires that the same PN code sequence be generated in the receiver and correlated with the received signal to extract the data modulation. One type of correlation technique employs a digital matched filter to compare the received digital signal with the locally generated version of the PN code. The digital filter produces an in phase (I) signal and a quadrature (Q) signal from which a digital demodulator such as a DPSK demodulator can derive data values. Another function of the digital matched filter is to produce correlation measurements from which synchronization signals can be generated.
In despreading a spread spectrum signal, the receiver produces a correlation pulse in response to the received spread spectrum signal when the received spread spectrum signal matches the chip sequence to a predetermined degree. Various techniques are available for correlating the received signal with the chip sequence, including those using surface acoustic wave (SAW) correlators, tapped delay line (TDL) correlators, serial correlators, and others.
Synchronization of signals between a transmitter and receiver that are communicating with each other in a spread spectrum communication system is an important aspect of the process of transmitting signals between them. Synchronization between transmitter and receiver is necessary to allow the despreading of the received signals by a spreading code that is synchronized between them so that the originally transmitted signal can be recovered from the received signal. Synchronization is achieved when the received signal is accurately timed in both its spreading code pattern position and its rate of chip generation with respect to the receiver""s spreading code.
A common problem encountered in most types of communications systems, particularly spread spectrum communications systems, is the dynamic nature of the channel. In many cases, the characteristics of the channel are not constant and changes with time. Some prior art communications systems employ some form of training sequence at the beginning of the packet before data reception begins. As a result, the receiver is dynamically adjusted for conditions on the channel that exist at the time of the training sequence. Further adaptation to the channel, however, is not performed for the remainder of the packet. In some cases, the packet may have a relatively long duration. In this case, depending on the changes that occur to the channel during reception, the receiver may lose synchronization and from that point onward would not be able to correctly receive data.
Therefore, it would be desirable if the receiver were able to dynamically adapt itself to varying conditions of the channel in such a way that the accuracy of the receiver is maintained. Having the ability to track changes in the channel improves the performance and reliability of the receiver.
The present invention is a direct sequence spread spectrum receiver for use in a communication system that utilizes a modulation technique referred to as code shift keying (CSK). Use of CSK modulation increases the number of bits transmitted per symbol, decreases synchronization requirements and improves performance. Code shift keying modulation transmits data in the form of circularly rotated spreading waveforms. The spreading waveform may comprise any type of waveform that has suitable auto correlation properties such as a chirp or PN sequence. The receiver of the present invention may be constructed to use any type of spreading waveform including a chirp or PN sequence.
During each symbol period (referred to as a unit symbol time or UST), a plurality of bits are transmitted. The symbol period is divided into a plurality of shift indexes with each shift index representing a particular bit pattern. The waveform is rotated by an amount in accordance with the data to be transmitted. The data is conveyed in the amount of rotation applied to the chirp before it is transmitted. Alternatively, the data may be conveyed in the shift differential between consecutive symbols. In addition to the rotation applied to symbols, the phase of the rotated spreading waveform is used to convey an additional bit of information.
The receiver is adapted to receive the signal transmitted through the channel. The receiver operates in either the acquisition mode or the tracking mode. During acquisition, the receiver attempts to acquire synchronization of the signal. It searches for the presence of a preamble and once found tries to synchronize on a synchronization sequence sent by the transmitter. The synchronization sequence is used by the receiver to train the receiver to the conditions on the channel and optionally to generate the initial value for the template to be used during the tracking mode of operation. The training sequence used for synchronization may or may not be the same training sequence used to generate the initial value for the tracking template.
During tracking mode, the receiver decodes the received symbols yielding the original transmitted data. The input frequency range of the receiver is divided into one or more frequency bands. For each frequency band, the receive signal is filtered, digitized and split into I and Q data streams wherein the Q data stream is delayed by xc2xcƒc. Both I and Q data streams are sampled and correlated with an adaptive template.
Correlations is used to detect the amount of rotation in the symbol. The received data is clocked into a shift register and circularly rotated. For each rotation shift, the correlator generates a correlation sum. The correlation output of both I and Q channels are summed over all the frequency bands and input to a maximum correlation detector. A shift index is chosen during each UST corresponding to the shift index that yields the maximum correlation sum. This shift index represents the original rotation applied to the transmitted symbol. The shift index is then decoded to yield the received data.
A key feature of the present invention is the dynamic adaptation of the template to the varying conditions of the channel. The template used to correlate the received symbol is continuously updated at each sample time using a template adaption function. The function performs an averaging of the previous contents of the template with the received symbol.
The transmitter sends data in the form of packets to the receiver. A start of packet field is placed at the beginning of the packet. The receiver searches for a correlation peak over all the possible shifts of each symbol received using linear correlation. Once the start of packet field is detected the receiver searches for a known pattern from the transmitter. Synchronization is achieved when the pattern is received. Once synchronization is achieved, cyclic correlation is used to receive the remainder of the packet. The data sent by the transmitter is encoded in the rotations applied to each symbol.
The CSK communication system has the advantages of higher reliability of transmission, simple and fast synchronization and immediate recovery from severe fading. In addition, a plurality of bits can be sent per symbol, permitting either a longer time duration for each symbol or a higher data throughput rate using the same symbol time duration as in typical direct sequence spread spectrum communications systems. Another advantage of the system is that it provides robustness against channels characterized by frequency varying signal to noise ratios. Further, the present invention can be implemented at low cost as a VLSI integrated circuit or ASIC.
There is therefore provided in accordance with the present invention a receiver for use in a code shift keying spread spectrum communication system comprising a front end circuit coupled to a channel and adapted to filter a receive input signal into N frequency bands, the front end circuit operative to generate N receive sample streams, each receive sample stream associated with one of the frequency bands, N receiver circuits coupled to the front end circuit, each receiver circuit associated with a frequency band, each receiver circuit comprising a template comprising a plurality of taps, initialization means operative to generate an initial value for the template from a plurality of training symbols stripped of rotation and phase whereby the template is initialized to the channel, means for generating receive symbols from the receive sample stream, a correlator adapted to correlate the contents of the template with the received symbol, the correlator operative to generate a correlation sum for each relative rotation between the template and the received symbol, for all possible transmitted rotations of a symbol, a combiner for combining the correlation sums generated by the N receiver circuits so as to yield a total correlation value, a maximum correlator detector operative to determine a shift index corresponding to a maximum total correlation value over all rotations of the template, the shift index subsequently decoded to yield receive data and wherein N is a positive integer.
There is also provided in accordance with the present invention, in a code shift keying spread spectrum communications system, a method of receiving, the method comprising the steps of filtering an input signal received from a channel into N frequency bands and generating N receive sample streams therefrom, each receive sample stream associated with one of the frequency bands, for each frequency band providing a template comprising a plurality of taps, generating an initial value for the template from a first plurality of de-rotated and phase stripped training symbols thereby adapting the template to the channel, synchronizing a symbol clock in response to a second plurality of training symbols, generating receive symbols from the receive sample stream in accordance with the symbol clock, correlating the contents of the template with the received symbol so as to generate a correlation sum for each relative rotation between the template and the received symbol, for all possible transmitted rotations of a symbol, combining the N correlation sums so as to yield a total correlation sum, determining a shift index corresponding to a maximum total correlation sum over all rotations of the template, decoding the shift index to yield receive data and wherein N is a positive integer.
There is further provided in accordance with the present invention a code shift keying spread spectrum communication transceiver comprising a transmitter adapted to generate symbols for transmission over a channel, the symbols comprising a spreading waveform circularly shifted in accordance with the data to be conveyed by the symbol and a receiver comprising a front end circuit coupled to a channel and adapted to filter a receive input signal into N frequency bands, the front end circuit operative to generate N receive sample streams, each receive sample stream associated with one of the frequency bands, N receiver circuits coupled to the front end circuit, each receiver circuit associated with a frequency band, each receiver circuit comprising a template comprising a plurality of taps, acquisition means operative to generate an initial value for the template from a plurality of training symbols that have been stripped of rotation and phase whereby the template is adapted to the channel, means for generating receive symbols from the receive sample stream, a correlator adapted to correlate the contents of the template with the received symbol, the correlator operative to generate a correlation sum for each relative rotation between the template and the received symbol, for all possible transmitted rotations of a symbol, a combiner for combining the correlation sums generated by the N receiver circuits so as to yield a total correlation sum, a maximum correlator detector operative to determine a shift index corresponding to a maximum total correlation sum over all rotations of the template, the shift index subsequently decoded to yield receive data and wherein N is a positive integer.